How AIDS Works

A man with AIDS is comforted by his wife as he lies in his bed at Zimbabwe's Matibi Mission Hospital, where at the time of the photo nearly 60 percent of the hospital's cases were AIDS related. Gideon Mendel/Getty Images

Like all viruses, HIV treads the fine line that separates living things from nonliving things. Viruses lack the chemical machinery that human cells use to support life. So, HIV requires a host cell to stay alive and replicate. To reproduce, the virus creates new virus particles inside a host cell, and those particles carry the virus to new cells. Fortunately the virus particles are fragile.

Viruses, including HIV, don't have cell walls or a nucleus. Basically, viruses are made up of genetic instructions wrapped inside a protective shell. An HIV particle, called a virion, is spherical and has a diameter of about one 10,000th of a millimeter.

HIV infects one particular type of immune system cell. This cell is called the CD4+T cell, a type of white blood cell also known as a T-helper cell. In fact, the virus only targets a subset of the T-helper cells: those that have already been exposed to infection. This is because, unlike "naive" cells, the experienced "memory" cells are in constant motion, and HIV uses that motion in a complex way to get inside them. Once infected, the T-helper cell turns into an HIV-replicating cell. T-helper cells play a vital role in the body's immune response. There are typically 1 million T-cells per 1 milliliter of blood. HIV will slowly reduce the number of T-cells until the person develops AIDS.

HIV is a retrovirus, which means it has genes composed of ribonucleic acid (RNA) molecules. Like all viruses, HIV replicates inside host cells. It's considered a retrovirus because it uses an enzyme, reverse transcriptase, to convert RNA into DNA [source: Lu et al.].

To understand how HIV infects the body, let's look at the virus's basic structure:

Viral envelope: This is the outer coat of the virus. It's composed of two layers of fatty molecules, called lipids. Embedded in the viral envelope are proteins from the host cell. There are also about 72 copies of so-called Env protein, which protrudes from the envelope surface. Env consists of a cap made of three or four molecules called glycoprotein (gp) 120 and a stem consisting of three to four gp41 molecules.

p17 protein: The HIV matrix protein lies between the envelope and core. It's a structural protein that plays multiple roles in the life cycle of the HIV virus, including viral replication and particle assembly. P17 also acts as a viral cytokine, a substance that helps cells communicate and move in a particular direction [source: Fiorentini et al.].

Viral core: Inside the envelope is the core, which contains 2,000 copies of the viral protein p24. These proteins surround two single strands of HIV RNA, each containing a copy of the virus's nine genes. Three of these genes — gag, pol and env — contain information needed to make structural proteins for new virions.